From: Vallery Stanishev (vall@physto.se)
Date: Tue Dec 09 2003 - 02:43:05 PST
Dear All,
The document I wrote was not meant to be a complete description
of the reduction or complete analysis. And apparently there
are some points that are not clear.
About Rachel's comment #2. Yes, I first flat-field the images,
and then subtract the background and extract the spectra.
About #3. Before combining Riess' images I shifted them
according to what is given in the fits headers. Here by
'shift' I do mean sub-pixel shift. I did this with IRAF's
IMSHIFT, which allows sub-pixel shifts.
Perhaps this is not the best way to handle this, but since
the software to deal ACS/grism dithered images is not
out yet I used IRAF.
About the dithering, sub-pixels, etc.. Riess' images were
dithered by a few pixels only. On all 16 images the SN was
within a square of 20x20 pixels. I also don't know if this
is large enough to start seeing differences in the wavelength
calibration (but this may be tracked down form the field dependence
of the wavelength solution given in the aXe's conf. file).
My guess would be that this 20-pixels shift is
fairly small to affect the wavelength calibration and hence
the flux calibration of Riess' spectrum.
I agree that the point of doing sub-pixel dithering is
to improve the resolution. If the software to handle such
data is about to be released we can use sub-pixel dithering.
However, I'm not quite sure that the spectral resolution is
of big concern for us. Given the broad SN features, especially
at z>1, a spectral resolution of ~90A seems to be enough to type the
object and to get the redshift with reasonable accuracy.
Moreover, for these very faint high-redshift SNe the S/N
will not be high and we shouldn't hope to get something
more than that. I would be more concerned about bad pixels
and in this aspect I think that integer pixels dithering
should be enough.
Regards,
Vallery
Rachel G. wrote:
>>As Vallery points out, there is a discrepancy with Adam's spectrum,
>>especially on the red end. Please send comments to Vallery and cc to
>>this mail list - hstsearch@lbl.gov.
>
>
> Two immediate comments on Vallery's conclusions
> regarding Riess' data:
>
> 1. We should be able to handle sub-pixel dithered data. It
> isn't an easy problem, but there is existing software that
> can properly handle such data. I can look into this.
>
> 2. The background subtraction is an important step.
> Remember flat-fielding is wavelength dependent, so the
> background you subtract has to be flattened properly as
> well. This could very well be the source of some of the
> differences with Riess' reduction (along with #1).
>
> 3. The correlated noise you see in your shift and add
> of Riess' data cannot be directly compared to a straight
> sum. If you simply sum Riess' data ignoring image shifts,
> you should find the data have better SNR than ours (although
> they could have been taken when background levels were
> higher). But even in the present sum, one can see Riess'
> spectrum has more interesting structure (features) than do
> ours.
>
> Definitely summing sub-pixel dithered data has to be
> handled more carefully.
>
> In addition, Given there looks to be a bit of a lull
> in the search planning (for a few days perhaps) I will
> finish my write-up of the reductions and analysis I did.
> The details of how this stuff is done is important and I
> suggest we compare results before moving on.
>
> Rachel
>
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